The effect of chlorine on β-D-galactosidase activity of sewage bacteria and Escherichia coli was studied. β-D-galactosidase activity of sewage was more resistant to chlorine than faecal coliform cultivability. At low initial dosage (0.05 mg Cl_2 l~(-1)) neither cultivability (colony-forming units (cfu)), nor enzyme activity of E. coli suspensions were severely impaired. When initial chlorine concentration was increased to 0.1 mig Cl_2 l~(-1), the cuf number decreased whereas enzyme activity remained high, i.e. the enzyme activity calculated cfu~(-1) increased. At higher chlorine doses both cfu and enzyme activity were reduced, but non-cultivable cells retained assayable activity after chlorination. Mean values of the enzyme activity calculated cfu~(-1) decreased when the chlorine dosage was increased from 0.1 to 0.5 mg Cl_2 l~(-1), but were not significantly different (P > 0.05) for dosages of 0.2-0.7 mg Cl_2 l~(-1). After chlorination, β-D-galactosidase activity of E. coli was less reduced than cfu and direct viable count numbers, but more reduced than 5-cyano-2-3, ditolyl tetrazolium chloride and total cell counts, and the enzyme activity represented an alternative activity parameter of chlorinated samples.
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